(a) Field of the Invention
The present invention relates to a liquid measurement instrument for continuously analyzing particular ingredients in a liquid by using electrochemical analysis and, more particularly, to a liquid measurement instrument suitable for analyzing body fluid such as human blood or other fluids.
(b) Description of the Related Art
A liquid measurement (test) instrument is generally used for analyzing ingredients in body fluid such as human blood. Patent Publication JP-A-1(1989)-101968 describes a conventional test instrument wherein a chemical sensor is used for testing blood ingredients extracted or sampled from a living body.
FIG. 1 shows the conventional test instrument as mentioned above, wherein a catheter 51 for sampling human blood is connected to the test instrument through a connector 54 attached to a first end of the test instrument by a tube 52. The test instrument comprises a chemical sensor 55, a first 3-way cock 56, a second 3-way cock 57, a pressure sensor 58, a flow control valve 59 connected by pipe 53 and arranged in this order from the connector 54 toward the second end of the test instrument to which a reservoir 61 for receiving therein a reference liquid 60 is connected. A pH sensor, a partial-pressure sensor for detecting PO.sub.2 or PCO.sub.2, an electrolyte sensor for detecting Na, K, Cl or Mg, a protein sensor, a blood sugar sensor or other sensor is used as the chemical sensor 55 depending on the use of the test instrument.
The first 3-way cock 56 is disposed between the chemical sensor 55 and the second 3-way cock 57, with a branch port of the first 3-way cock 56 being connected to a syringe 62. The first 3-way cock either connects the catheter 51 with the second 3-way cock 57, with the syringe 62 being disconnected, or connects the catheter 51 to the syringe 62. The second 3-way cock 57 is disposed between the first 3-way cock 56 and the pressure sensor 58, with a branch port thereof being open to the drain. The second 3-way cock either passes the reference liquid 60 to the chemical sensor 55 or drains the reference liquid 60 from the chemical sensor 55. The flow control valve 59 codes a glass pole 59a having a small axial hole and an elastic tube 59c press-fit onto the glass pole 59a and slidably mounted on the pipe 53. When the elastic tube 59c is thrust in a direction normal to the axial direction, a space is provided between the inner wall of the elastic tube 59c and the external wall of the pipe 53, thereby bypassing the reference liquid 60 during a flashing operation.
For flashing the pipe 53, after the catheter 51 is removed from a living body, the reservoir 61 is filled with the reference liquid 60, followed by thrusting the elastic tube 59c to form a bypass for introducing a comparatively large quantity of the reference liquid 60 into the pipe 53. After the reference liquid 60 is drained from the catheter 51 together with air bubbles to clean both the catheter 51 and the pipe 53, the catheter 51 is inserted in the blood vessel of the living body while both the catheter 51 and the pipe 53 are filled with the reference liquid. During this operation, both the 3-way cocks 56 and 57 are set for passing the reference liquid 60 to the catheter 51 through the pipe 53 as shown in FIG. 1.
Then, the second 3-way cock 57 is switched for closing the pipe 53, as shown in FIG. 2, to calibrate the sensor by using the reference liquid 60 and the chemical sensor 55. The calibration data is read through signal lines 64 extending from the chemical sensor 55 by a microcomputer not shown.
Subsequently, the first 3-way cock 56 is switched so that the piston 62a in the syringe 62 is pulled out to introduce the reference liquid 60 into the syringe 62 and introduce blood 63 into the pipe 53 through the catheter 51 by using a negative pressure, as shown in FIG. 3. The chemical sensor 55 disposed for a particular ingredient detects the ingredient in blood 63 and supplies data to the microcomputer.
After the measurement is completed, the blood 63 is returned to the living body by thrusting the piston 62a of the syringe 62. Patent Publication JP-A-62(1987)-24139 describes an automated calibration of a test instrument with a simple configuration.
Referring to FIG. 4, the test instrument described in JP-A-62(1987)-24139 comprises a first reservoir 91 for receiving a carrier liquid 81, a second reservoir 101 for receiving a first reference liquid 82 comprising the carrier liquid and a calibrating substance and a third reservoir 111 for receiving a second reference liquid 83 comprising the carrier liquid and a disturbing substance. Tubes extending from the reservoirs 91, 101 and 111 are connected to an 4-way cock 84 for controlling the direction of liquid flow. The 4-way cock 84 is driven by a servomotor and controlled by an encoder for selecting a desired liquid flow. One port of the 4-way cock 84 is connected to a cell (reactor) 85 having a sensor 87 with a tube 121, and a pump 86 is provided for supplying the liquids from the reservoirs to the cell 85.
FIG. 5 shows a partially cutout perspective view of the cell 85, wherein a sample liquid is supplied through an inlet port 132 and discharged hugh an outlet port 133. Since the outlet port 133 is disposed at a higher position than the inlet port 132, a specified amount of sample liquid stays within the cell 85, with the excess liquid or measured liquid overflowing through the outlet port 133 to a waste tank 88. The sensor 87 comprises thee electrodes including an enzyme electrode 87a attached with a living catalyst such as enzyme, a disturbance electrode 87c for detecting a disturbance substance and a counter electrode 87b. The three electrodes 87a, 87b and 87c are connected to respective external electrodes by connectors 87d. On the top of the cell 85, an elastic cap 131 is inserted in the central area, and a liquid inlet 131a is provided in the center of the elastic cap 131a for supplying sample liquid. The 4-way valve 84 may be such that shown in FIG. 6 instead, wherein the 4-way valve 84 in FIG. 4 is implemented by three individual electromagnetic valves 84a.
Utility Model Publication JM-A-7(1995)-56001 describes a sampling instrument such as shown in FIG. 7. The sampling instrument is dedicated for sampling and comprises a cell (housing) including a funnel area 153, having a sampling port 159 communicated to a suction port 151 with a valve 157, for collecting a body fluid 161 from a living body. In operation, body fluid is first collected by vacuum and introduced to a storage space 160 of the valve 157, then a handle 156 of the valve is operated to align the storage space 160 with an outlet port 165 for takeout of the body fluid. The takeout can be conducted without stopping the vacuum pump or removing a cover.
FIG. 8 shows a modification of the sampling instrument of FIG. 7. The sampling instrument of FIG. 8 is similar to the sampling instrument of FIG. 7 except for a pair of storage spaces 160 provided in a valve 157 and a pair of outlet ports 165 in the modification. In operation, body fluid is introduced to one of the storage spaces 160 from the sampling port 159, then the handle 156 is operated to align the one of the storage spaces 160 to one of the outlet ports 165 and to align the other of the storage spaces 160 with the sampling port 159. This enables to obtain a sampling operation and a takeout operation simultaneously for improvement of the throughput.
The conventional measurement instruments as described above have the following problems.
First, sampling is interrupted in the measurement instruments during cleaning the cell by a buffer solution after measurement and subsequent supplying a new buffer solution into the cell. The sampling is also interrupted by removing a sample liquid from the cell after measurement.
Second, a large quantity of sample liquid must be sampled for measurement because a sensor is installed in a chamber, which results in a large quantity of dead volume.
Third, an inaccurate measurement is caused by the precedent sampling liquid remaining within the cell after the cleaning thereof.
Fourth, the sensor calibration is limited by the facts that the reference liquid cannot be supplied into the cell when both ports are closed for the sampling period and that the sensor must be cleaned by a buffer solution after removing the sample liquid. Especially, cleaning is difficult because of the column configuration of the cell.